that is pretty much the first piece of mathematics any student
learns. They are used to count, initially to count things,
concrete objects such as pennies or marbles. This is in some respects
surprising, since pennies and marbles are never really identical.
In physics, however, one encounters particles that are -
electrons, for example, differ only in their position or orientation.

The natural numbers are usually defined along with a set of operations
known as arithmetic2 . The well-known
operations of arithmetic are addition, subtraction, multiplication, and
division. One rapidly sees that the set of natural/counting numbers is
not closed with respect to them. That just means that if one
subtracts 7 from 5, one does not get a natural number; one cannot take
seven cows away from a field containing five cows, one cannot remove
seven pennies from a row of five pennies.

Natural numbers greater than 1 in general can be factored into a
representation in prime numbers3 . For
example:

(2)

or

(3)

This sort of factorization can sometimes be very useful, but not so much
in introductory physics.